Compilation fails when used as an ESP-IDF component (HardwareSerial maybe-uninitialized)

The baud rate detection introduced by https://github.com/espressif/arduino-esp32/pull/1961 causes a compiler error when arduino is being used as an esp-idf component:

```
error: 'detectedBaudRate' may be used uninitialized in this function [-Werror=maybe-uninitialized]
```